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Big Bang and GR

  1. Dec 1, 2004 #1
    Hi Everybody,

    I hope i'm not being too simplistic and I don't want to stir up a religious debate. We have just derived Einstein's G Law in my graduate course and after seeing how many assumptions we make, it's amazing that so much of our understanding of physics is based on GR. I am left with more questions than answers.

    Anyway, as we extrapolate to some "point" in the past; I think that matter, with its immense density, would behave as a black hole right?

    But, as we approach a black hole's event horizon doesn't time go to infinity? How do we arrive at a "age of the universe". Why is the Big Bang the most widely accepted theory? With our current understanding (which is pretty much nothing) of what goes on within a black hole there is absolutely no way to falsify this theory it seems.

    Let us have a good discussion.

  2. jcsd
  3. Dec 1, 2004 #2


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    Welcome DF! The big bang singularity [which probably was not a true singularity] is a subject of much interest. It does not, however, behave in the same manner as a 'classical' black hole. At the instant of the big bang, the 'rules of engagement' are not yet in place. The separate forces that govern the current universe were unified [so it is thought] and the energy levels were far beyond anything that has existed since that time. When inflation kicked in [according to current theory], the universe expanded so rapidly that gravity [weakling that it is] was a helpless bystander until it was too big for gravity to reel it back in.
  4. Dec 1, 2004 #3


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    Joe (DiracFeynman)
    I dont know that the universe had very much matter in it at the epoch when expansion started----not as much as it does today anyway.
    If you extrapolate backwards and concentrate matter you would overestimate the density back then. The catch is that there was inflation.

    Inflation creates matter. Inflation scenarios are merely that: scenarios.
    But they are widely accepted and they do create matter----seemingly in defiance of conservation.
    The scalar field that drives inflation is like a stronger version of the
    "dark energy" that we seem to be observing today---the simplest model for it is just a constant energy density.

    Any constant or nearly constant energy density has a negative pressure and has an expansive effect on space----and if the density is, say, one joule per cubic meter then with every new cubic meter volume created a new joule of energy comes into existence. Finally when there is a big volume there has been created a lot of energy.

    You can try to balance the books by introducing negative gravitational energy but the people who do this kind of scenario-theorizing often do not even bother to balance the books. They are like those talented corporate financial officers one hears about, so few of whom actually go to jail.

    Anyway the Beginning Picture that people describe does not look like a whole Universefull of matter squeezed into a tiny cubbyhole.
    It is more like a moderate amount of matter, dominated by one of these scalar fields (an almost constant energy density, gradually changing but acting roughly like an extra strong dark energy density).
    And the scalar field is strong enough to win and expansion wins and is
    actually really fast---big exponential growth in a fraction of a second

    and then the scalar field "thermalizes" and turns to a muck of more familiar particles. they in turn begin to slow expansion down, but by then space has expanded and is rapidly expanding and the ordinary matter is thinned out so it cant slow things down very much very quickly.
  5. Dec 1, 2004 #4


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    Dearly Missed

    Looking back, I see Chronos already responded, and much more concisely.
    He makes a good point. there are several parts to the story. I will leave it for you to sort out.
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